The goal of this project is to identify mechanisms mediating effects of endogenous TGF-"Symbol"B1 on the development and function of specific hematopoietic cell lineages. Experimental evidence from in vitro studies of function suggests a critical role for TGF-"Symbol"B1 in regulating myelopoiesis and immune cell function, but such studies are limited by their inability to reproduce the complex in vivo network which ordinarily influences the genesis and function of these cell populations. Our approach to this problem has been to utilize the TGF-"Symbol"B1-null mouse as the basis for establishing complimentary in vivo model systems. Each is designed to isolate TGF-"Symbol"B1 deficiency in a background that allows for determination of the significant growth-inhibitory and tumor suppressor functions of the molecule in the adult mouse. This approach has enabled us to identify critical links between cell cycle control by TGF-"Symbol"B1, immune cell maturation and function, and hematopoietic cell growth and differentiation, as they occur in vivo. Examples include our demonstration of myeloid hyperplasia as a distinct phenotype in TGF-"Symbol"B1-null mice, and of the contributions of several regulators of the cell cycle to the establishment of the autoimmune process which evolves in the absence of endogenous TGF-"Symbol"B1. Through adoptive transfer studies, including bone marrow transplant experiments and studies of thymic and splenic reconstitution in nude mice, we have also identified novel autocrine effects of TGF-"Symbol"B1 and their roles in the autoimmune and hematopoietic manifestations of TGF-"Symbol"B1 deficiency. Through a collaboration with the Dermatology Branch in the DCS, we have determined that TGF-"Symbol"B1 mice lack epidermal Langerhans cells (LC) and gp40 positive dendritic cells (DC) in lymph nodes, suggesting that lymphoid DC as well as LC are perturbed. We have now established stem cell factor-dependent stem cell lines from these mice, providing an in vitro system in which we may study the mechanisms by which TGF-"Symbol"B1 regulates development of both lymphoid and myeloid lineages. Our goal is to define the cellular and molecular events that govern stem cell differentiation and commitment, as well as growth arrest in response to TGF-"Symbol"B. In addition, we have utilized this approach to create similar stem cell lines from a single yolk sac of mice carrying mutations in critical, pathway restricted Smad signaling intermediates. These lines will be invaluable in assessing the TGF-"Symbol"B/bone morphogenetic protein (BMP)-specific effects on hematopoietic development and reconstitution. Studies such as immune reconstitution in chimeric mouse models and in vitro assays of differentiation are currently underway and promise to provide mechanistic insights into how this family of proteins controls blood cell development and function.